The acquisition of physical measured values, especially pressures, frequently requires that such values are ascertained in or on available or installed equipment. Sometimes this factor makes it problematic to find suitable connection points for the attachment of the transducer devices. Frequently, it happens that such connecting points are exposed to adverse environmental influences or conditions which adversely affect the useful life of the transducer devices or which at least make the precision of the transducers less reliable. Thus, it is practical to sense the physical values to be measured in one location where the measured value is also converted into an electrical value and to then further process the respective electrical value or values at a more suitable location. The just mentioned approach has its limitation in that difficulties will arise especially when a direct electrical connection between the measuring transducers and the signal processing devices is not possible. This difficulty applies in connection with equipment in which direct electrical connections are too trouble prone, for example, due to technical sealing problems, due to mechanical stress and/or vibrations, or due to thermal conditions. Difficulties with establishing direct electrical connections also occur especially in machines or at least machine components in which the measuring transducer location is movable relative to an indicator or display of the measured value or relative to a data processing device.
A measuring device of the type here involved is disclosed in German Patent Publication (DE-PS) 2,951,139 describing a device for the monitoring of the air pressure in vehicle tires A capacitive pressure sensor or transducer is located next to the rotating vehicle wheel, whereby any change in the tire pressure causes a respective change in the capacity of the capacitive sensor. Such a capacitive pressure sensor or transducer constitutes a frequency determinining structural circuit component of a multi-vibrator, the oscillating frequency of which becomes a measure for the tire pressure to be measured. Such a multi-vibrator comprises in addition to the sensor capacity an inductive coil, which together with the measuring or sensing capacity constitutes a pressure sensing structural component, referred to herein simply as a pressure sensor, which is so arranged that with each wheel revolution the pressure is passing, with a narrow spacing, an inductive coil forming part of an evaluating device, which receives the frequency converted measuring value signals and converts these signals into corresponding values, which in turn represent a measure for the tire pressure. In the just mentioned conventional example, it is, however, not necessary that the measured air pressure is ascertained with a special accuracy because the conventional device is intended to merely make sure that the tire pressure does not fall below a certain threshold value. However, the known pressure sensor would lead to substantial measuring errors, if the ambient temperature near the measuring capacitor should change substantially. This is so, because the mechanical dimensions of the pressure sensor alone would change due to heat-caused material expansions. In order to eliminate such external influences on the sensing capacitor, a substantial structural effort and expense would be necessary. However, even if such expense should be acceptable under certain circumstances, most of the time, there is not enough space for the installation of the sensor or it cannot be installed due to weight limitations which must be met by such a pressure sensor.